Here's an overview of the main methods used in DAC:

Chemical Absorption Methods 

Chemical absorption, particularly using liquid solvents, is one of the most developed DAC technologies:

1. Amine-based solvents:  
• Highly selective for CO2 
• Can achieve high capture rates 
• Challenges include energy-intensive regeneration and potential for solvent degradation 
 

2. Alkali hydroxide solutions (e.g., sodium hydroxide): 
• Forms stable carbonate precipitates 
• Requires high-temperature regeneration 
• Offers potential for high-purity CO2 streams 
 

Physical Adsorption Methods

Physical adsorption uses solid sorbents to capture CO2:

1. Activated carbons: 
• High surface area and porosity 
• Relatively low cost 
• Moderate CO2 selectivity 
 

2. Zeolites: 
• Highly porous aluminosilicate materials 
• Good CO2 selectivity, especially at low temperatures 
• Sensitive to moisture 
 

3. Metal-Organic Frameworks (MOFs): 
• Highly tunable pore structures 
• Potential for high CO2 capacity and selectivity 
• Still in research and development phase 
 

Moisture Swing Adsorption

This innovative approach uses anionic exchange resins:

• Absorbs CO2 when dry and releases it when moist 
• Utilizes the latent heat of water phase change 
• Requires further research for cost-effectiveness 
 

Membrane Separation 

Emerging technology using semi-permeable membranes: 

• Requires minimal water usage 
• Smaller footprint compared to other methods 
• Still in development, with a focus on improving permeability and selectivity 
 

Process Configurations 

DAC systems typically employ cyclic processes for continuous operation:

1. Temperature Swing Adsorption (TSA): 
• Uses heat for sorbent regeneration 
• Suitable for both chemical and physical sorbents

2. Pressure/Vacuum Swing Adsorption (PSA/VSA): 
• Utilizes pressure changes for CO2 capture and release 
• Often combined with TSA for enhanced performance 
 

3. Electric Swing Adsorption (ESA): 
• Uses electric current for direct heating of sorbents 
• Potential for improved energy efficiency

Challenges and Ongoing Research 

1. Reducing the energy required for sorbent regeneration and air movement 

2. Improving CO2 capacity, selectivity, and durability 

3. Enhancing heat integration and reducing operational costs 

4. Addressing engineering challenges for large-scale deployment 
 

Future Prospects 

As research progresses and costs decrease, DAC technologies are expected to play an increasingly important role in achieving net-zero emissions. The U.S. Department of Energy's Carbon Negative Shot initiative aims to drive innovation in this field, targeting CO2 removal at gigatonne scales for less than $100 per net metric ton of CO2-equivalent.

By combining DAC with long-term CO2 storage or utilization, these technologies offer a promising pathway for actively reducing atmospheric CO2 concentrations and mitigating climate change impacts.